Processes for cleaning a cathode tube and assemblies in a hollow cathode assembly

ABSTRACT

“The present invention is a process for cleaning a cathode tube and other subassemblies in a hollow cathode assembly. In the disclosed process, hand covering elastomer gloves are used for handling all cathode assembly parts. The cathode tube and other subassemblies are cleaned with a lint-free cloth damped with acetone, then wiped with alcohol, immersed in ethyl alcohol or acetone, and ultrasonic agitation is applied, heating to 60° C. for ethyl alcohol or 56° C. for acetone. The cathode tube and other sub assemblies are dried by blowing with nitrogen gas.”

This is a divisional of application Ser. No. 09/152,407 filed Sep. 14,1998 now U.S. Pat. No. 6,064,156.

ORIGIN OF THE INVENTION

The invention described herein was made by employees of the UnitedStates Government and may be manufactured and used by or for thegovernment for government purposes without payment of any royaltiesthereon or therefore.

FIELD OF THE INVENTION

The present invention relates to a hollow cathode electron source, whichfinds application in plasma generation, and more particularly to along-life hollow cathode plasma generator and to its design,manufacturing process and assembly. These manufacturing processesinclude contamination control procedures which cover hollow cathodecomponent cleaning procedures, gas feed system designs andspecifications, and hollow cathode activation and operating procedures.

BACKGROUND OF THE INVENTION

Cathodes emit electrons when elevated in temperature by a process knownas thermionic emission. Thermionic emitters generally consist of a wirethat is made of some refractory metal, which may be typically made oftungsten or molybdenum. The wire is then coated, or impregnated withsome low work function material, such as barium carbonate, andsubsequently ohmically heated.

Hollow cathodes have been in existence for over ten years. Hollowcathodes have been developed to an advanced state of technologyreadiness for ion propulsion. Ionic propulsion may be defined aspropulsion by the reactive thrust of a high-speed beam of similarlycharged ions ejected by an ion engine. In ground tests they havedemonstrated high emission currents of greater than 30 Amperes, and longlifetimes, with modest power requirements of less than 100 Watts. Hollowcathode plasma sources have demonstrated versatile and effectiveoperation as plasma contactors in ground testing of various devices.This testing includes plasma bridge neutralizers for ion thrusters,plasma contactor demonstration experiments for the electrodynamictether, and space station structure potential control experiments.

Hollow cathodes have also been flown in space as components of ionpropulsion systems and spacecraft charging/charge-control systems,including ATS-6, SERT-II, SCATHA, and SCSR-1 flight experiments.Demonstrated capabilities in space tests include lifetimes of 10,000hours and more than 300 restarts. NASA flight experiments havedemonstrated hollow cathode plasma contactors to be effective incontrolling both the negative charging and differential charging of thespacecraft frame. Hollow cathodes have been operated in space under avariety of orbital and environmental conditions; on spacecraft,including an Agena vehicle, on communication satellites, and on thespace shuttle. Environments include those of low-earth orbits,sun-synchronous high inclination orbits, and geosynchronous orbits.

All of the above hollow cathode development was accomplished withmercury as the hollow cathode expellant, or “working fluid.” For avariety of reasons, which includes spacecraft contamination, the presenthollow cathodes preferentially use an inert gas, such as xenon, as theexpellant. Subsequent to the transition from the use of mercury to xenonin the early 1980's, there have been, and continue to be, failures ofhollow cathodes in the United States, in Europe, and in Japan. Thesehave impacted both research and development activities and flightprograms. The failures have apparently been primarily due to inadequateprocedures and protocols to control contamination during thefabrication, assembly, testing, storage, handling, and operation of thecathodes; as well as, inadequate design and process features. To date,the only successful extended duration tests, that have been reported ofusing inert-gas hollow cathodes at high emission currents of greaterthan 1 Ampere, have been conducted, by the NASA Lewis Research Center.These successful extended duration tests were implemented by the use ofthe design features and processes that are further described herein.

U.S. Pat. No. 3,944,873, granted Mar. 16, 1976, to J. Franks, et al.,discloses a hollow cathode of cylindrical shape. A cathode encloses ananode having a pair of screen electrodes, symmetrically disposed aboutand parallel to the plane of the anode. The anode has a central apertureand another aperture may be made in the cathode diametrically oppositethe first aperture.

U.S. Pat. No. 4,049,989, granted Sep. 20, 1977, to R. H. Bullis, et al.,discloses ion production using a permeable electrode having aperturesand a central electrode. A wire mesh grid is placed symmetrically aboutthe permeable electrode.

U.S. Pat. No. 4,087,721, granted May 2, 1978, to G. Mourier, disclosesan ion source that is comprised of a hollow cathode dischargearrangement having an anode placed between two cathodes. The cathode hasholes through which some of the ions of the plasma escape.

U.S. Pat. No. 4,377,773, granted Mar. 22, 1983. to A. Hershcovitch. etal., discloses an ion source that is comprised of a hollow cathode andan anode base having electrically connected anode covers.

U.S. Pat. No. 4,428,901, granted Jan. 31, 1984, to W. H. Bennettdiscloses a hollow cathode, that is held inside of a cathode holder, aswell as, a hollow anode that is supported by a conducting support. Adiode envelope surrounds the hollow cathode.

U.S. Pat. No. 4,894,546, granted Jan. 16, 1990, to R. Fukui et al.,discloses a cylindrical hollow cathode having upper and lower circularanodes that are placed at the two ends of the cylindrical cathode, whereeach of the anodes have circular openings.

U.S. Pat. No. 5,075,594, granted Dec. 24, 1991, to R. W. Schumacher, etal., discloses a hollow cathode used for discharging ionized plasma ofan ambient gas, such as xenon. A flat anode extends perpendicular to,and is intersected by, the axis of the cathode. A keeper/baffleelectrode, which may also be a plate, is disposed between the cathodeand anode. Even though this device is a low impedance device, it willnot yield electron emission currents to an external electrode in themulti-ampere range, within a voltage range of 20 Volts.

U.S. Pat. No. 5,241,243, granted Aug. 31, 1993, to G. Cirri, discloses aplasma generator that is comprised of a hollow cylindrical cathode andone or more anodes.

U.S. Pat. No. 5,352,954, granted Oct. 4, 1994, to G. Cirri, discloses aplasma generator that is comprised of a hollow cylindrical cathode andone or more anodes having holes.

U.S. Pat. No. 5,569,976, granted Oct. 29, 1996, to N. V. Gavrilov, etal., discloses of an ion emitter that is comprised of a hollow cathodeat one end and a coaxial rod-shaped anode at the other end. The hollowcathode encloses the rod shaped anode.

U.S. Pat. No. 5,581,155, granted Dec. 3, 1996, to A. I. Morozov, et al.,discloses a plasma accelerator that is comprised of a hollow cathode andan annular anode.

All of the above referenced prior art relate to high voltageacceleration systems. Further, they do not teach of a self-regulatingemission control system. None of the prior art relates to an ionicemission apparatus, having low current capability, with the exception ofU.S. Pat. Nos. 5,075,594 and 4,428,901, which disclose the use ofelectron emission apparatus. Only U.S. Pat. No. 5,075,594, teaches of alow output impedance, whereas all of the others have an undesirable highoutput impedance, that is not suitable for use in space stationapplications.

In addition, none of the above referenced prior art provide an attainedperformance reliability having a demonstrated lifetime in excess of thepresent state-of-the-art 500 hours, when operated at emission currentsof approximately 1 Ampere.

These devices in the past have exhibited unstable operatingcharacteristics and shortened lifetimes as a result of design andprocessing problems. Until the initiation of the present program, therehave been no inert gas hollow cathodes that had demonstrated lifetimesgreater than 500 hours, when operated at emission currents greater than1 Ampere.

The present invention differs from the aforementioned prior art inasmuchthat the approach is not limited solely by the design of the apparatusbut also includes the method of manufacturing processes and procedure inorder to obtain a highly reliable and repeatable design commensuratewith a high life expectancy. The advancements demonstrated in themanufacturing processing include contamination control procedures whichcover hollow cathode component cleaning procedures, gas feed systemdesigns and specifications, and hollow cathode activation and operatingprocedures.

Accordingly, it is therefore an object of the present invention toprovide an electron emissive hollow cathode apparatus that will providereliable, stable and repeatable operation over a lifetime that is inexcess of 17,500 hours.

It is another an object of the present invention to provide an electronemissive hollow cathode apparatus that will provide reliable, stable andrepeatable operation over a broad range of operating emission currentsof at least a 6:1 ratio.

It is still another an object of the present invention to provide anelectron emissive hollow cathode apparatus that will provide reliable,stable and repeatable operation, while permitting electron emissioncurrents of up to 30 Amperes emission to an external anode, at voltagesof less than 20 Volts DC.

It is a final object of the present invention to provide a method ofmanufacturing an electron emissive hollow cathode apparatus that whenadhered to, will provide reliable, stable and repeatable operation overan expected lifetime that is in excess of 17,500 hours.

These as well as other objects and advantages of the present inventionwill be better appreciated and understood upon reading the followingdetailed description of the presently preferred embodiment taking inconjunction with the accompanying drawings.

SUMMARY OF THE INVENTION

The present invention relates to the design and processes that arerequired to fabricate long lived hollow cathode assemblies, which willexhibit stable and repeatable operating parameters.

These processes have been demonstrated for emission currents up to 24Amperes and lifetimes greater than 10,000 hours, and they have beenincorporated in a cathode design which is proposed for controlling thefloating potential of International Space Station Alpha (ISSA). Thesedesign and processes permit stable and repeatable operation over a broadrange of emission currents under variable and uncertain current demand,at electron emission currents up to 30 Amperes at potentials of lessthan or equal to 20 Volts, and with a life expectancy of at least 17,500hours.

The International Space Station Alpha (ISSA) power system is designedwith high voltage solar arrays which operate at output voltages oftypically 140-160 Volts, and is configured with a “negative ground” thatelectrically ties the habitat modules, structure, and radiators to thenegative tap of the solar arrays. This electrical configuration and theplasma current balance that results will cause the habitat modules,structure, and radiators to float to voltages as large as 120 Volts withrespect to the ambient space plasma. As a result of these large negativefloating potentials, there exists the potential for deleteriousinteractions of ISSA with the space plasma. These interactions mayinclude arcing through insulating surfaces and sputtering of conductivesurfaces due to the acceleration of ions by the spacecraft plasmasheath. Both of these processes may result in changes in the surfacematerial properties, in destruction of coatings and in contamination ofthe surfaces due to redeposition.

The space experiment SAMPIE (for Solar Array Module Plasma InteractionsExperiment) was recently flown on the Space Transportation System STS-62and provided valuable validation of the theoretical models of spacecraftcharging that were used to predict the station floating potentials. Theflight data that was acquired from this experiment, which quantified thecurrent collection to station solar array elements, confirmed the needfor a plasma contactor to control the potential of the space station.

A decision was made, that was based on its potential effectiveness, tobaseline a plasma contactor system on ISSA as the solution to alleviateplasma interactions. Consequently, NASA initiated a plasma contactordevelopment program as a portion of the ISSA electrical power system.

There are several major derived operational requirements for the stationplasma contactor system which include: (a) the capability to controlstation potential to within 20 Volts of space plasma potential; (b) emitelectron currents up to 30 Amperes under dynamic and variableconditions; (c) operate for up to 17,500 hours without degradation; (d)minimize consumables; (e) be single-fault tolerant in design; (f) becompatible with all space station utilities; (g) be roboticallyserviceable; and (h) incorporate health monitoring instrumentation,including instrumentation to measure the plasma return current.

For the ISSA application, efficient and rapid emission of high electroncurrents is required by the plasma contactor system under conditions ofvariable and uncertain current demand. A hollow cathode assembly is wellsuited for this application and was therefore selected as the criteriafor the design of the station plasma contactor system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective cutaway view of the Plasma Contactor System(PCS), which incorporates the use of the Hollow Cathode Assembly.

FIG. 2 is a perspective cutaway view of the space station Hollow CathodeAssembly (HCA).

FIG. 3 is a sectional view of the space station Hollow Cathode Assemblygiving the functional attributes.

FIG. 4 is a sectional view detailing the components of the space stationHollow Cathode Assembly.

FIG. 4A is an enlarged sectional view showing the cathode tube, posts,seal fitting as taken through section 4A—4A of FIG. 4.

FIG. 4B is an enlarged cross section view showing the cathode tube,posts and mounting adapter taken through section 4B—4B of FIG. 4A.

FIG. 5 is a sectional view of the cathode tube and swaged heatersub-assembly.

FIG. 6 is an enlarged sectional view of the seal fitting, isolator tube,screen, and Xenon feed line.

FIG. 7 is a schematic view of the “idle” mode of operation, where thesolar panel arrays are eclipsed.

FIG. 7A is a schematic view that diagrammatically illustrates the “idle”mode of operation.

FIG. 8 is a schematic view that illustrates the “clamping” mode ofoperation, where the solar panel arrays are fully illuminated.

FIG. 8A is a schematic view diagrammatically illustrates the “clamping”mode of operation.

FIG. 9 is a sectional view of the keeper sub-assembly with keeper capand keeper tube.

FIG. 10 is an enlarged view of the disk orifice plate.

FIG. 11 is a sectional view of the disk orifice plate taken through11—11 of FIG. 10.

FIG. 12 is an enlarged sectional view of the cathode tube.

FIG. 13 is an end view of the cathode tube; and,

FIG. 14 is an end view of FIG. 9, the keeper sub-assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The design and manufacturing process of the present invention wasdeveloped to produce Hollow Cathode Assemblies that could operate over abroad range of emission currents up to 30 Amperes, at low potentials,with lifetimes of at least 17,500 hours.

Hollow cathode assemblies can vary in overall size, cathode heatingmethod and operation depending on their operating requirements, feed gasand intended application. These devices have been implemented ascomponents of ion thrusters, as well plasma contactors for spacecraftcharge control. These assemblies are also used for material processessuch as thin film deposition and ion beam milling.

The present invention, by using the design and associated processes asdescribed herein, produces cathode assemblies that have stable andrepeatable operating conditions of both discharge current and voltage. Alifetime of greater than 10,000 hours, and having an expected orprojected lifetime of greater than 17,500 hours was demonstrated, wherethe present state-of-the-art is less than 500 hours at emission currentsin excess of 1 Ampere. Stable operation can be provided over a largerange of operating emission currents, up to a 6:1 ratio, and can emitelectron currents of up to 30 Amperes in magnitude to an external anode(simulating the current drawn to a space plasma) at voltages of lessthan 20 Volts.

FIG. 1 is the preferred embodiment of the ISSA Plasma Contactor System10. The Plasma Contactor System (PCS) 10 is comprised of foursubsystems: they are, a Hollow Cathode Assembly (HCA) 20, a PowerElectronics Unit (PEU) 30, an Expellant Management Unit (EMU) 40, and anOrbit Replaceable Unit (ORU) 50.

FIG. 2 is a perspective view of the completed Hollow Cathode Assembly(HCA) 20, one of the subsystems of the ISSA Plasma Contactor System(PCS), shown with the cathode tube 130, xenon feed line 137, swagedheater 138, connective wires 142 stabilized in wire clamp 142 a, of theHCA, that mounts to the PCS by mounting flange 158.

The Hollow Cathode Assembly 20 (HCA), as shown in the sectional view ofFIG. 3, is the active electron emitter source. A cover sheath 129stabilizes a hollow cathode tube 130 in fixed position within thesheath. The HCA is further comprised of a hollow cathode 130, a low-workfunction insert 123 for electron emission, an anode 100, a heater 138,and an electrical isolator 128. The present invention is of anenclosed-keeper geometry, that operates using xenon gas. The over-ridingcriteria used in the design of an HCA is one that satisfies thepotential control requirement of less than a 20 Volt clamping voltageand one that maximizes the expectations for long life. Based on theserequirements, an enclosed-keeper hollow cathode geometry was used.

Xenon gas is introduced into the Hollow Cathode Assembly 20 via thexenon feed line 137. The flow of the pressurized xenon gas passesthrough the expellant isolator tube 136 and subsequently through screen135. The expellant isolator tube 136 provides the electrical isolationof the cathode common from the spacecraft ground to permit the directmeasurement of the emission current.

The xenon gas then flows into the Hollow Cathode 130, where the cathodeis electrically isolated from the anode by electrical isolator 128. Thecathode insert 123 is the active electron emissive source. The anodeassembly 100 sustains the operation of the Hollow Cathode 130. Thesheathed heater 138 raises the cathode temperature for activation andignition of the Hollow Cathode Assembly 20. The entire HCA assemblymounts to the side wall of the Plasma Contactor System (PCS) 10, via thecathode assembly mounting flange 158. Keeper cap 147, keeper tube 148and keeper collar 149, are also disclosed in FIG. 3.

Turning now to FIG. 4, the HCA design of the present invention iscomprised of a hollow cathode 130, an anode 100, a heater 138, anexpellant isolator tube 136, and an electrical isolator 128. The cathodeassembly mounting flange 158 secures the HCA assembly to the side wallof the Plasma Contactor System (PCS) 10.

The hollow cathode itself is comprised of a refractory alloy tube 130with an orifice plate 127 welded on one end. The tube is severalmillimeters in diameter, while the orifice 127 a, (shown in FIG. 10), inthe plate 127 is a fraction of a millimeter in diameter. An insert 123,which is located within the hollow cathode, serves as a low-workfunction electron source, and is electrically connected to the tube 130.A heater 138, used to raise the temperature of the cathode duringactivation and to facilitate ignition, surrounds the downstream end ofthe hollow cathode tube 130.

The HCA incorporates a cylindrical anode 100 that surrounds the hollowcathode 130 that is in close proximity. The anode 100 is also referredto as the keeper as it maintains, or keeps the cathode emitting duringconditions when there is no external emission current demanded. Theanode 100 is described as being enclosed because it completelyencapsulates the hollow cathode except for a single aperture 180, founddirectly downstream of the hollow cathode orifice 127 a found in orificeplate 127. This design is more efficient than an open-keeper geometrybecause of the higher internal pressures in the cathode-keeper region.

Upstream from the hollow cathode tube 130 is an expellant isolator tube136 that isolates the HCA from the ISSA structure. This allows theconnection from the HCA cathode to the station single point ground to bemade via cable, and this current can then be monitored byinstrumentation within the power electronics unit.

An engineering model design of the HCA has been completed and severalunits have been fabricated and are under test. The mass of the HCA, lessthe power cable and connector, is approximately 125 grams (4.4 oz.). Itis cylindrical in shape, and has a length of approximately 11.5 cm. andmaximum diameter of approximately 2.8 cm. The HCA has three interfacesto the Plasma Contactor System (PCS) 10 of FIG. 1, including amechanical attachment point to the Orbital Replaceable Unit (ORU) 50, asingle xenon gas line 137 to the Expellant Management Unit (EMU) 40, anda 3-conductor electrical cable to the Power Electronics Unit (PEU) andController 30.

In reference to FIG. 4A, an enlarged sectional view taken through 4A—4Aof FIG. 4, there is further disclosed an active electron emitter insert123, the cathode tube 130, leg projections 131, seal fitting 132, sealfitting nut 132 a, mounting adapter 133, leg holder 134, screen 135, andcompression fitting adapter 157.

Referring to FIG. 4B, an enlarged cross section view of the cathode tubetaken through section 4B—4B of FIG. 4A, there is shown cathode tube 130,leg projections 131, mounting adapter 133.

FIG. 5, a sectional view of the cathode tube and swaged heatersub-assembly, better discloses disc orifice plate 127, swaged heater138, radiation shield 139, and connective cable 142.

FIG. 6 is an enlarged sectional view of the expellant isolatorsub-assembly showing seal fitting 132, screen 135, expellant isolatortube 136, and the xenon feed line 137.

Referring to FIGS. 7, 7A, 8 and 8A, the active emission to the spaceplasma of electron current at least matching the net electron currentcollected on the station solar arrays, more commonly referred to as the“clamping” mode, is required for only a small portion of the stationorbital period. This period occurs during approximately one-third of theorbit, from dawn through noon when the solar arrays are illuminated,generate power, and face in the ram direction. During the remainder ofthe orbital period the hollow cathode plasma source may be eitheroperated in an “idle” mode or turned off. Both the idle-mode andclamping-mode of operation are illustrated in FIGS. 7 and 7A, and 8 and8A, respectively.

The critical and distinctive features of the fabrication and assemblyprocesses of the cathode assembly include (1) the cathode tube cleaningprocess, (2) the assembly of the cathode and (3) the heater fabrication,assembly, test and inspection processes.

Hollow Cathode Assembly Fabrication and Assembly

The cathode assembly design details, manufacturing processes, materialhandling, and rigorous acceptance testing are further described in thefollowing paragraphs. Failure to follow these steps may result in acathode assembly that has unsteady voltage-current characteristics andhas a relatively short life expectancy.

Preliminary Cleaning Procedure:

The Hollow Cathode Assembly (HCA) cleaning procedure of metal partsafter component fabrication and prior to assembly is as follows:.

1. Hand contact will be only with use of hand-coverings, such asunpowdered latex or nitrile gloves.

2. Clean any residual dirt or grease from the part first by wiping itwith a clean, lint-free cloth or tissue and reagent grade acetone, then95% ethyl alcohol.

3. Completely immerse the part to be cleaned in a beaker, that isimpervious to alcohol and to acetone and place the beaker in anultrasonic cleaner, having a heater.

4. Place the part in an ultrasonic cleaner that is under a fume hood.Start agitation and heater of the ultrasonic cleaner. Agitate and heatfor 30 minutes; starting from when the solution reaches a hightemperature of either 56 degrees C. when using acetone or 60 degrees C.when using ethyl alcohol. Re-position the part as necessary using cleantweezers to ensure that the part is completely immersed.

5. Remove the part from the beaker or container and drain. The partshould be handled only with tweezers.

6. Then, place the part in a clean beaker and fill with clean 95% ethylalcohol. Completely immerse the part.

7. Repeat steps 3 through 5 using 95% ethyl alcohol.

8. Blow dry with clean, oil-free nitrogen.

9. Store the cleaned part in a nitrogen-purged, marked hermeticallysealed plastic bag, using a hermetic bag sealer or equivalent.

1.0 Cleaning Procedures for the Cathode Tube and Disc OrificeSub-assembly

The cathode tube is shown in FIGS. 12 and 13; and the disk orifice platein FIGS. 10 and 11.

NOTE:

Use clean powder-free latex examination gloves when handling all HCAparts. Steps 3.6-3.8 in this procedure requires that the cathode tubesub-assembly(ies) be submitted for vacuum firing.

1.1 Clean any residual dirt or grease from the interior of the cathodetube sub-assembly. Run a clean, lint-free cloth strip dampened withreagent grade acetone in and out of the cathode tube sub-assembly untilno dirt or grease appears on the cloth. Inspect the inner cathode tubeusing a Mini-Mag Lite to confirm that there is no visible dirt orparticulate contamination. Then blow the sub-assembly dry withultra-high purity nitrogen.

1.2 Repeat 1.1 using clean 95% ethyl alcohol.

1.3 Clean cathode tube sub-assembly following the Preliminary CleaningProcedure.

1.4 Store the cathode tube sub-assembly(ies) in a properly labeled,nitrogen-purged hermetically sealed bag(s) until the vacuum firingprocedure is conducted.

1.5 Using a vacuum furnace, vacuum fire each sub-assembly. Pump down thefurnace to a vacuum level of less than 1×10⁻³ torr. Pre-heat thefurnace. Ramp at 10 degrees F./minute to 2000 degrees F.; hold for 30minutes, then ramp to 2200 degrees F. at 10 degrees F./minute; hold foran additional 30 minutes. Turn off heat and cool to room temperaturebefore venting.

2.0 Hollow Cathode Assembly (HCA) Assembly Procedures

NOTE:

All assembly procedures must be conducted in a Class 1000 clean room ona cleaned stainless steel or formica clean room-rated table

2.1 Cathode Tube Sub-Assembly

FIGS. 12 and 13 best illustrate the cathode tube sub-assembly. Theinsert cathode sub-assembly 123 is procured as a completed sub-assembly.The cathode tube 130 is welded to the disc orifice plate 127 and vacuumfired.

2.2 Swaged Heater Sub-Assembly Fabrication and Installation

With reference to FIG. 5, install the swaged heater 138 onto the cathodetube 130 using Saureisen Electrotemp, Cement #8.

2.3 Keeper Sub-Assembly Fabrication and Assembly

As shown in FIGS. 9 and 14, the keeper sub-assembly 126 is comprised ofkeeper cap 147 having aperture 180, and keeper tube 148; the keepercollar 149, is shown in FIG. 3.

2.4 Expellant Isolator Sub-Assembly Preparation

Referring to FIG. 6, the expellant isolator is shown in section. Theexpellant isolator sub-assembly 125 is procured as a brazedsub-assembly. It is comprised of xenon feed line 137, nut 154, expellantisolator tube 136, seal fitting 132 and screen 135.

2.5 Keeper Brazing Sub-Assembly Brazing Procedures

Braze the sleeve 141 to the swaged heater sheath 138; the swaged heatersheath 138, the cathode tube 130, and the keeper collar 149, to theelectrical isolator 128; and the keeper tube 148 to the keeper collar149, shown in FIG. 3.

2.6 Anode Connector Assembly

The e-beam weld technician shall insert the anode connector 150 into thekeeper collar 149.

2.7 Insert Cathode Sub-Assembly Installation Procedure

Install insert cathode sub-assembly 123 into cathode tube 130, buttingthe insert cylinder securely against the back of the disc orifice plate127, while maintaining the position of the insert legs at 120 degreeintervals, with no cross-over. Install leg holder 157, so that theinsert legs slide into the slots in the leg holder and that the upstreamplane of the leg holder is flush with the upstream end of the cathodetube 130.

2.8 Expellant Isolator Sub-Assembly Installation Procedure

Install expellant isolator sub assembly 125 onto cathode tube 130 whilebeing sure that the cathode tube is fully butted against the expellantisolator sub-assembly.

3.0 Swaged Heater Compaction Test Procedure

Remove all burrs from the edges from the swaged heater samples using afine-tooth file. Wipe the sheaths clean with lint-free tissue. Measurethe mass of the test samples with the analytical balance to within1×10⁻³ grams. Place test samples in a clean beaker and completelyimmerse the samples in kerosene for a minimum of 24 hours.

Remove the test samples from the beaker using clean tweezers. Carefullywipe off any residual kerosene from the sheath surface using a lint-freetissue only. Remove residual kerosene from the magnesium oxide byplacing the cloth onto the kerosene and allowing it to absorb theliquid, while not making contact with the magnesium oxide. Measure themass of the test samples with the analytical balance to within 1×10⁻³grams. Remove the magnesium oxide from each sample by placing the samplein a vice with cleaned flat, smooth surfaces and partially flatteningthe outer sheath. Loosen the vice, rotate the sample 90 degrees anddeform the sheath again.

Remove the sample and dislodge the loose magnesium oxide with a 0.020inch diameter drill bit (avoid removing tantalum material). Repeat thisprocess until the center conductor can be pulled out and all themagnesium oxide can be remove from the sheath. Remove residual magnesiumoxide by rinsing the sheath and center conductor in clean ethyl alcohol,blowing compressed gas through the sheath interior, and by wiping thesheath and center conductor. Measure the mass of the sheath and centerconductor with the analytical balance to within 1×10⁻³ grams.

Calculate the compaction percentage of each sample using the followingequation:${\% \quad {Compaction}} = {\frac{M_{pre} - M_{{sh}/w}}{\frac{M_{post} - M_{pre}}{D_{kero}} + \frac{M_{post} - M_{{sh}/w}}{D_{MgO}}} \times \frac{1}{D_{MgO}}}$

where D_(kero)=0.820 grams/cm. and D_(MgO)=3.58 grams/cm.

Swaged heater compaction shall be 84%±4%. Reject the heater it fails tomeet this compaction range.

3.1 Swaged Heater Thermal Imaging Scan Procedure

Suspend the swaged heater using the alligator clips attached to thecenter conductor at either end to support the heater. Be sure that theswaged heater does not make contact with any object. Contact withanything will create a heat conduction path that will invalidatetemperature distribution results.

Using a Kikusui Power Supply Model PAD55-6L, or equivalent, connect thepositive terminal to one end of the swaged heater center conductor andconnect the opposite end of the swaged heater to a Fluke Model 77 SeriesII Multimeter, or equivalent, and to the negative terminal of the powersupply to measure the heater current.

Orient the Inframetrics Model 600 Thermal Imaging Radiometer, orequivalent so that the temperature distribution through out the entirelength of the swaged heater can be scanned. Set the emissivity to 1.0.Use a black cloth as a black background to reduce background emissions.

With the power supply in current control mode, raise the current to 2.40Amperes. Wait for approximately 10 minutes for the swaged heatertemperature to rise to approximately 17-23 degrees C. Using the thermalimaging radiometer, measure the swaged heater temperature andtemperature along the length of the swaged heater sheath.

The temperature distribution along the length of the swaged heater shallbe less than ±5 degrees C.

The critical and distinctive test processes which insure cathodeassembly longevity include contamination control procedures that areimplemented for: (1) the expellant feed system, (2) the cathode insertactivation sequence, as well as (3) the cathode ignition.

4.0 Xenon Feed Systems Test Procedures

This procedure is for the design and implementation of a Hollow CathodeAssembly (HCA) xenon feed system to satisfy the specified contaminationlevels and calibration procedure requirements, to maintain compatibilitywith long-life HCA operation.

Secure all components, including the xenon gas bottle and any extendedlengths of gas tubing, to a gas panel to preclude relative motion andmitigate loosening of fittings due to vibration or bending moments.

Conduct an xenon feed system bake-out, as described in 4.2.

Conduct a leak rate determination, as described in 4.3.

After the xenon feed system has been exposed to the vacuum facilitywhich is at <1.3×10⁻⁴ Pa (<1.0×10⁻⁶ torr) for a minimum of 12 hours,close the pressure regulator, the capacitance manometer shut-off valve,the purge line shut-off valve, and all metering valves. Open the xenonbottle valve fully. Pressurize the xenon feed system to 20-30 psig byadjusting the second stage pressure of the pressure regulator.

4.1 Hollow Cathode Assemblies Laboratory Xenon Feed System Calibration

The following procedures provide accurate calibrations of heat transfertype flow transducers for determination of true xenon volumetric flowrates at standard temperature and pressure (i.e. 0 degrees C. and 101325N/m², respectively), assuming the use of a simple ‘bubble-calibration’technique. This technique provides repeatable and accurate calibrationdata when compared to data obtained using primary standards whichimplement more sophisticated measurement techniques. These proceduresare anticipated to be transportable to calibration of the xenonvolumetric flow rates of the flight xenon feed system.

Conduct the following calibration:

Calibrate the flow meter/controller over a range of flow rates of 4.0 to8.0 sccm, in increments of 0.5 sccm using the following equation:${{True}\quad {{Flow}\quad\lbrack{sccm}\rbrack}} = {\frac{V}{t} \times \frac{P - P_{v}}{P_{std}} \times \frac{T_{std}}{T}}$

where:

V=glass tube volume, cc=10 cc

t=time to fill volume, minutes

P=barometric pressure, in. of Hg

P_(v)=vapor pressure of water at the ambient temperature and barometricpressure, in. of Hg

P_(std)=standard pressure, 29.92 in. of Hg

T_(std)=standard temperature, K=273.14 K

T=xenon temperature [assumed to be equal to the ambient temperature], K

Plot the true flow rate as a function of the indicated flow rate and uselinear regression to obtain a calibration equation for the flow meter.

4.2 Hollow Cathode Assembly (HCA) Laboratory Xenon Feed System HighTemperature Bake-out.

The following procedure should be conducted whenever a pressurizedportion of the xenon feed system has been exposed to air (e.g. afterassembly, a xenon bottle replacement, feed system component replacement,etc.).

Expose the xenon feed system to a vacuum facility held at a pressure ofless than 1.3×10⁻⁴ Pa (1.0×10⁻⁶ torr).

These procedures are implemented to remove adsorbed oxygen andoxide-bearing compounds (moisture, etc.) from the internal surfaces ofthe feed system lines and components. Adsorption of these constituentsoccurs when the interior of the system is exposed to atmosphere (such asduring modifications to the feed system including xenon bottle and feedsystem component replacements). Out-gassing of these adsorbedconstituents into the xenon gas stream during operation of the hollowcathode assembly may result in contamination of the cathode insert andrapid failure of the hollow cathode assembly. These procedures, whenimplemented, have demonstrated more than 18,000 hours of xenon hollowcathode operation.

Monitor the feed system temperature with a Type K thermocouple atvarious locations. Gradually increase the heater tape input power untila temperature range between 50 and 120 degrees C. is reached at variouslocations on the feed system. Maintain these temperatures for 24 hours.Upon completion, allow the xenon feed system to cool to room temperaturewhile under pressure.

4.3 Leak Rate Determination of Xenon Feed Systems.

Conduct this procedure whenever a pressurized portion of the xenon feedsystem has been exposed to air (e.g. after feed system assembly, a xenonbottle replacement, feed system component replacement, etc.).

The following procedure describes a rate-of-rise test which is used todefine both the out-gassing rate of adsorbed constituents from feedsystem interior surfaces and the leak-integrity of the xenon feed systemto cross-diffusion of atmospheric gases.

Close only the valves that expose the xenon feed system to the vacuumfacility and ensure that all other valves, including the pressureregulator, are open: then start data acquisition. Conduct this test fora minimum of 24 hours. At the completion of the test, stop dataacquisition and expose the xenon feed system to the vacuum facilitywhich is at <1.3×10⁻⁴ Pa (<1.0×10⁻⁶ torr) by opening all closed valves.The leak rate is determined using the following equation:${{Leak}\quad {{Rate}\quad\lbrack{sccm}\rbrack}} = {\frac{V}{RT} \times \frac{p}{t} \times F_{convert}}$

where:

V=volume of feed system exposed to capacitance manometer

R=Specific Gas Constant of air [287 J/kg-K]

T=average ambient temperature throughout test [K dp/dt slope [Pa/sec j]

F_(convert)=Conversion Factor=4.98×10′ [sccm-sec/kg]

The leak rate shall be less than 1.5×10⁻⁵ sccm.

Continue to evacuate the feed system for a minimum of 12 hours.

4.4 Determination of Xenon Gas Purity

The following is a listing of elements known to be detrimental to HCAlifetime and performance.

Non-Metals: Carbon, Oxygen, Chlorine, Fluorine, Phosphorous and Sulfur

Alkali Metals: Sodium and Potassium

Transition Metals: Titanium, Zirconium, Hafnium, Chromium, Manganese,Iron, Cobalt, Gold, Silver, Platinum and Zinc

Other Metals: Silicon, Tin, Antimony, Lead, and Bismuth

The measured xenon purity shall be greater than 99.999%.

The xenon shall have the following measured impurity levels:

Less than 0.1 ppm Oxygen (O₂)

Less than 0.1 ppm Water (H₂O)

Less than 0.5 ppm Carbon Monoxide (CO)

Less than 0.5 ppm Carbon Dioxide (CO₂)

Less than 0.1 ppm Carbon Tetrafluoride (CF₄)

Less than 0.1 ppm Total Hydrocarbons

Less than 1 ppm Nitrogen (N₂)

Less than 2 ppm Hydrogen (H₂)

Less than 5 ppm Krypton (Kr)

Less than 1 ppm Argon (Ar)

Balance Xenon

Flow xenon from the xenon feed system into the sample bottle at 6 sccm.When the fill is complete, ship the sample bottles to the vendor foranalysis.

4.5 Conditioning the impregnated insert of the Hollow Cathode Assembly.

These procedures are to be conducted subsequent to every exposure of theHCA to air, at pressures above 1.3×10⁻² Pa (1.0×10−4 torr) and prior toignition. An oil-free facility with a base pressure of <6.7×10⁻⁴ Pa(<5.0×10⁻⁶ torr) is required.

4.5.1 Procedure

All procedures are to be conducted while maintaining a facility pressureof <6.7×10⁻⁴ Pa (<5.0×10⁻⁶ torr).

Install the HCA in a vacuum of <5.0×10⁻⁶ for at least 12 hours (to allowfor outgassing of the HCA and its insert) prior to initiation of theconditioning sequence.

Energize the heater to 3.85 Amperes having a corresponding criticaltemperature of 550° C. for a minimum of 3 hours.

De-energize the heater for half an hour.

Energize the heater to 7.2 Amperes having a corresponding criticaltemperature of 550° C. for 1 hour.

De-energize the heater for a minimum of at least one-half hour.

4.6 Procedure necessary for ignition of the gaseous electrical dischargebetween cathode and anode electrodes of the Hollow Cathode Assembly.

The HCA must be conditioned per the procedure in the preceding section4.5 prior to ignition. If the HCA has been exposed >1.3×10⁻² Pa (1×10⁻⁴torr) at any time subsequent to conditioning, the conditioning proceduremust be repeated before an ignition may be attempted.

These procedures are to be initiated at a pressure <6.7×10⁻⁴ Pa (5×10⁻⁶torr).

4.6.1 Requirements:

4.6.1.1 Ignition pulse generator circuit requirements:

The ignition pulse generator output shall have a magnitude of 750±100Volts with a leading-edge rate-of-rise of >150 V/microsecond.

The ignition pulse duration shall be less than 20 microseconds.

The ignition pulse generator circuit output shall have a frequency of 10Hz.

The pulse ignitor shall be active on power up of the anode power supply.When the anode current of 0.5 Amperes is established, the charging stageof the ignition pulse generator shall be disabled until the circuitcurrent is extinguished. The ignitor shall automatically engage uponinvoluntary discharge extinction.

4.6.1.2 Anode power supply stage requirements:

The anode power supply stage shall be provided, having nominalcharacteristics of +40 VDC at 3.0 ADC current.

4.6.1.3 Heater power supply requirements:

The heater power supply shall be energized until the anode currentexceeds 2.5 ADC.

4.6.2 Procedure

Confirm that the HCA has not been exposed to air at pressures >1.3×10⁻²Pa (1×10⁻⁴ torr) at any time subsequent to the most recent conditioningsequence. If this cannot be confirmed, a conditioning must be performedper the preceding section

4.5, before an ignition may be attempted.

These procedures are to be conducted at pressures <6.7×10⁻⁴ Pa (5×10⁻⁶torr) prior to gas flow, and at pressures of >6.7×10⁻⁴ Pa (5×10⁻⁶ torr)during gas flow, as long as this pressure rise is entirely a result ofthe gas flow.

4.6.2.1 HCA ignition sequence:

At t=0 seconds: Apply 8.5 DC Amperes, limited to 74.5 W, to the HCAheater.

At t=206 seconds: Open gas control valve to allow xenon flow.

At t=210 seconds: Energize the anode power supply to apply open circuitvoltage and engage the ignition pulse generator.

Maintain the heater current until an anode current of 2.5 Amperes isdetected. Ignition should occur prior to t=1800 seconds.

After 2.5 Amperes of anode current is detected, de-energize the heaterpower supply.

What is claimed is:
 1. A process for preliminary cleaning of eachmetallic part for assembly of a long-life hollow cathode assembly,comprising the steps of: a) using hand-coverings to eliminate handcontact with a hollow cathode assembly part; b) cleaning any residualdirt or grease from the part first by wiping it with a clean, lint-freeand reagent grade acetone, followed by 95% ethyl alcohol; c) immersingthe part to be cleaned completely in a beaker, said beaker beingimpervious to alcohol and to acetone, and said beaker containing asolution of acetone; d) placing the beaker in an ultrasonic cleanerhaving a heater under a fume hood; e) heating the ultrasonic cleaner for30 minutes, with agitation thereof when the solution of acetone reaches56 degrees C.; f) re-positioning the part as necessary in said beakerusing clean tweezers to ensure that the part is completely immersed; g)removing the part from the beaker using tweezers, and draining anymoisture from the part; h) immersing the part completely in a cleanbeaker, said beaker containing a solution of 95% ethyl alcohol; i)placing the beaker in an ultrasonic cleaner having a heater under a fumehood; j) heating the ultrasonic cleaner for 30 minutes, with agitationthereof when the solution of ethyl alcohol reaches 60 degrees C.; k)re-positioning the part as necessary in said beaker using clean tweezersto ensure that the part is completely immersed; l) removing the partfrom the beaker using tweezers, and draining any moisture from the part;m) drying the part with clean, oil-free nitrogen; and, n) storing thecleaned part in a nitrogen-purged, marked hermetically sealed plasticbag, for completely elimination of dust and debris that may contaminatea hollow cathode assembly part or subassembly.
 2. A process for cleaninga cathode tube and disc orifice subassembly of a Hollow Cathode Assembly(HCA), wherein hand contact includes use of hand-covering elastomergloves for handling all HCA parts, comprising the steps of: a) cleaningany residual dirt or grease from the interior of the cathode tubesub-assembly using a clean, lint-free cloth strip dampened with reagentgrade acetone in and out of the cathode tube sub-assembly until no dirtor grease appears on the cloth; b) inspecting the inner cathode tubeusing a Mini-Mag Lite to confirm that there is no visible dirt orparticulate contamination; and, c) blowing the sub-assembly dry withultra-high purity nitrogen.
 3. The process for cleaning a cathode tubeand disc orifice subassembly according to claim 2, further comprisingthe steps of: repeating the same steps of claim 9 a to 9 c using clean95% ethyl alcohol.
 4. The process for cleaning a cathode tube and discorifice subassembly according to claim 3, further comprising: a)immersing the subassembly in a beaker said beaker being impervious toalcohol and acetone, said beaker containing a solution of acetone; b)placing the beaker in an ultrasonic cleaner having a heater and a hood;c) positioning the beaker over the heaters, and under the hood; d)starting agitation of the beaker and its contents, and starting theheater of the ultrasonic cleaner.
 5. The process for cleaning a cathodetube and disc orifice subassembly according to claim 4, wherein step,starting agitation and starting the heater, includes the steps of: a)heating with agitation for 30 minutes to a temperature of 56 degrees C.;b) re-positioning the subassembly as necessary using clean tweezers toensure that the subassembly is completely immersed; c) removing thesubassembly from the beaker with clean tweezers; and, d) draining anyexcess fluid from the subassembly; e) placing the subassembly in a cleanbeaker and filing said beaker with clean 95% ethyl alcohol; f) immersingthe subassembly completely; g) repeating steps described in 9 (a to d)and 10 (a to d) using 95% ethyl alcohol; and, h) blowing dry thesubassembly with clean, oil-free nitrogen.
 6. The process for cleaning acathode tube and disc orifice subassembly according to claim 5, furthercomprising the step of: storing the cathode tube sub-assembly in aproperly labeled, nitrogen-purged hermetically sealed bag until a vacuumfiring procedure is conducted.
 7. The process for cleaning a cathodetube and disc orifice subassembly according to claim 6, furthercomprising the steps of: removing the cathode tube sub-assembly from theproperly labeled, nitrogen-purged hermetically sealed bag; firing eachsub-assembly by a vacuum firing in a vacuum furnace.
 8. The process forcleaning a cathode tube and disc orifice subassembly according to claim7, wherein the firing step includes the steps of: a) pumping down avacuum furnace to a vacuum level of less than 1×10⁻³ torr; b)pre-heating the furnace; c) ramping at 10 degrees F./minute to 2000degrees F.; d) holding for 30 minutes; e) ramping to 2200 degrees F. at10 degrees F./minute; f) holding for an additional 30 minutes; g)turning off heat; and, h) cooling to room temperature before venting.